CHAPTER 18 ACID-BASE EQUILIBRIA

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1 CHAPTER 18 ACID-BASE EQUILIBRIA 18.1 The Arrhenius definition classified substances as being acids or bases by their behavior in the solvent water. 18. All Arrhenius acids contain hydrogen and produce hydronium ion (H 3 O ) in aqueous solution. All Arrhenius bases contain an OH group and produce hydroxide ion (OH - ) in aqueous solution. Neutralization occurs when each H 3 O molecule combines with an OH - molecule to form molecules of H O. Chemists found that the H rxn was independent of the combination of strong acid with strong base. In other words, the reaction of any strong base with any strong acid always produced 56 kj/mol ( H -56 kj/mol). This was consistent with Arrhenius s hypothesis describing neutralization, because all other counter ions (those present from the dissociation of the strong acid and base) were spectators and did not participate in the overall reaction It is limited by the fact that it only classified substances as an acid or base when dissolved in the single solvent water. The anhydrous neutralization of NH 3 (g) and HCl(g) would not be included in the Arrhenius acid/base concept. In addition, it limited a base to a substance that contains OH in its formula. NH 3 does not contain OH in its formula but produces OH - ions in H O Strong acids and bases dissociate completely into their ions when dissolved in water. Weak acids only partially dissociate. The characteristic property of all weak acids is that a significant number of the acid molecules are not dissociated. For a strong acid, the concentration of hydronium ions produced by dissolving the acid is equal to the initial concentration of the undissociated acid. For a weak acid, the concentration of hydronium ions produced when the acid dissolves is less than the initial concentration of the acid a) Water, H O, is an Arrhenius acid because it produces H 3 O ion in aqueous solution. Water is also an Arrhenius base because it produces the OH - ion as well. b) Calcium hydroxide, Ca(OH) is a base, not an acid. c) Phosphoric acid, H 3 PO 4, is a weak Arrhenius acid. It is weak because the number of O atoms exceeds the ionizable H atoms by 1. d) Hydroiodic acid, HI, is a strong Arrhenius acid Only (a) NaHSO Barium hydroxide, Ba(OH), and potassium hydroxide, KOH, (b and d) are Arrhenius bases because they contain hydroxide ions and form OH - when dissolved in water. H 3 AsO 4 and HOCl a) and c) are Arrhenius acids, not bases (b) H O and (d) H NNH both are very weak 18.9 a) HCN(aq) H O(l) H 3 O (aq) CN - (aq) CN H3O K a [ HCN] b) HCO - 3 (aq) H O(l) H 3 O (aq) CO - 3 (aq) CO 3 H3O K a HCO 3 c) HCOOH(aq) H O(l) H 3 O (aq) HCOO - (aq) HCOO H3O K a HCOOH [ ] 18-1

8 18.53 The conjugate pairs are H S (acid)/hs - (base) and HCl (acid)/cl - (base). The reactions involve reacting one acid from one conjugate pair with the base from the other conjugate pair. Two reactions are possible: (1) HS - HCl H S Cl - and () H S Cl - HS - HCl The first reaction is the reverse of the second. To decide which will have an equilibrium constant greater than 1, look for the stronger acid producing a weaker acid. HCl is a strong acid and H S a weak acid. The reaction that favors the products (K c > 1) is the first one where the strong acid produces the weak acid. Reaction () with a weaker acid forming a stronger acid favors the reactants and K c < K c > 1: HNO 3 F - - NO 3 HF - K c < 1: NO 3 HF HNO 3 F a) HCl NH 3 NH 4 Cl - strong acid stronger base weak acid weaker base HCl is ranked above NH 4 in Figure and is the stronger acid. NH 3 is ranked above Cl - and is the stronger base. NH 3 is shown as a stronger base because it is stronger than Cl -, but is not considered a strong base. The reaction proceeds towards the production of the weaker acid and base, i.e., the reaction as written proceeds to the right and K c > 1. b) H SO 3 NH 3 - HSO 3 NH 4 stronger acid stronger base weaker base weaker acid H SO 3 is ranked above NH 4 and is the stronger acid. NH 3 is a stronger base than HSO - 3. The reaction proceeds towards the production of the weaker acid and base, i.e., the reaction as written proceeds to the right and K c > b a) NH 4 - HPO 4 NH 3 - H PO 4 weaker acid weaker base stronger base stronger acid K c < 1 This is because acid-base reaction favors stronger acid and base. b) - HSO 3 HS - H SO 3 S - K c < 1 weaker base weaker acid stronger acid stronger base a a) The concentration of a strong acid is very different before and after dissociation. After dissociation, the concentration of the strong acid approaches 0, or [HA] 0. b) A weak acid dissociates to a very small extent, so the acid concentration after dissociation is nearly the same as before dissociation. c) Same as (b), but the percent, or extent, of dissociation is greater than in (b). d) Same as (a) No, HCl and CH 3 COOH are never of equal strength because HCl is a strong acid with K a > 1 and CH 3 COOH is a weak acid with K a < 1. The K a of the acid, not the concentration of H 3 O in a solution of the acid, determines the strength of the acid. 18-8

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